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VTTC first light!

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James

Sun Apr 03 2011, 12:21AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

For whatever reason, I'd never tried building a Tesla coil of any sort before so I decided it was time I take a crack at it. Here's an early prototype of my 811A VTTC with a nice little blue streamer. I haven't done any tuning yet other than to prop the primary up and it sprang to life. This is only about 400V on the plate, I'll get it tuned and then try cranking up the juice a bit.

James

Sun Apr 03 2011, 01:00AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Holy crap!!! Stepping up to a MOT made for some serious improvement! I'm floored by how well this thing works, I'm wondering now if I can improve it further with a bit of tuning. If I crank up the variac on the MOT too far the plate starts glowing orange pretty quick, I may have to use a smaller one and pull some turns off the secondary to get it so I can run it for long periods. Still, wow! I'm amazed, first Tesla coil and first time building anything with a vacuum tube too. It's not often things just work that easily.

ScotchTapeLord

Sun Apr 03 2011, 02:04AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

James wrote ...

Holy crap!!! Stepping up to a MOT made for some serious improvement! I'm floored by how well this thing works, I'm wondering now if I can improve it further with a bit of tuning. If I crank up the variac on the MOT too far the plate starts glowing orange pretty quick, I may have to use a smaller one and pull some turns off the secondary to get it so I can run it for long periods. Still, wow! I'm amazed, first Tesla coil and first time building anything with a vacuum tube too. It's not often things just work that easily.

Congratulations! And you can always improve with tuning, but there comes a point when the improvements are too microscopic to see. :)
Well done.

Arcstarter

Sun Apr 03 2011, 03:13AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Congrats

As for heating, instead of reducing power, just tune! Tune everything, the tank cap, primary turns, feedback turns, coupling of both feedback and primary, RC gridleak circuit, topload... I have achieved 11 inch sparks with no color change of the plate. Another thing is that some tubes operate better with a certain frequency. For example, Dr. Spark claims that the sweet spot of an 833C is around 300KHz. I suppose 1.4MHz is around the sweet spot for the 811A, because previously i have only used secondaries with half the resonant frequency and i never got such good performance.

Here is the best example i can come up with. Here is the video i took of my 811A coil with 7 or so inch sparks:

After tuning, i got 4 more inches of spark and LESS heating. Tuning matters!

James

Sun Apr 03 2011, 06:54AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Man that's impressive! Do you have a schematic? While the initial results of my coil were astounding, I've been fiddling around with it and have been unable to get much improvement. As it stands, I can do maybe 3-4" streamers without the plate starting to glow. The only doorknob caps I have are 4700pF so I've got two in series to get 2350pF. I've tried one and three and both yielded worse results. My secondary is 13.6" of #28 wire wound on a 2" acrylic form and the primary is #14 on a 4.5" PVC form. Grid leak is a 10K adjustable resistor with a 1,000pF capacitor across it. For some reason it seems to make almost no difference whether the resistor is set to max or min or another capacitor is paralleled across it.

Matt Edwards

Mon Apr 04 2011, 05:47PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Nice! keep us updated as you get it better tuned. It really reminds me of how much I need to get mine up and running again. I'm replacing the dead 811a's with 572b's and the MOT with a 3400v transformer. Hopefully sometime soon...

What schematic did you follow? Or did you just wing it?

James

Mon Apr 04 2011, 07:12PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

I based it on information in Steve's VTTC FAQ as well as various odds & ends I found online and parts I had on hand. At this point I need to figure out why I'm not getting better performance. It worked so well the first time I powered it up but I've been unable to squeeze much more out of it. I'm also surprised that going from 1k to almost 10k on the grid leak resistor has almost no effect, nor does doubling the capacitor across that resistor. Various toploads do give a bit more spark, but seem to cause corresponding increased heating in the tube. I can't run more than about 2" streamers for more than a few minutes without the plate glowing orange. Arcstarter's coil is way more impressive.

I noticed last night that my secondary is wound in the opposite direction of the primary and feedback winding. Does this matter? I've got quite a bit of EE experience but I'm admittedly a noob when it comes to both tesla coils and tubes.

Nah

Mon Apr 04 2011, 07:14PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Is that a heathkit B+ eleminator in the back?

Paul

James

Mon Apr 04 2011, 09:04PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Nah wrote ...

Is that a heathkit B+ eleminator in the back?

Paul

It's a Heathkit IP-32 power supply, intended as a bench supply for building vacuum tube circuits. It has a 0-400V output, 0-100V negative output and a 6.3V heater output. Inside it's entirely tube based and has a pair of 6L6 power tubes in the regulator circuit. I'm finding that it's great as a (relatively) safe power source for initial checkout of the VTTC circuit prior to stepping up to something larger.

Matt Edwards

Mon Apr 04 2011, 11:02PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

The direction doesnt matter. Stick with a smaller topload. As for the resistor that is strange. I placed a 50w variable on mine and it does affect both the output and heating. I am also having some heating issues but not quite as bad as yours. Much like you I quickly slapped mine together with what I had handy and have not tuned it much yet. I've gotten up to 10" from mine with full power at the MOT. But the tubes get too hot after a few minutes. Keep in mind that the 811a's are only rated for about 1000-1200v max which is why I decided to switch to the 572b's rated 2500v or so.
Here is a quick vid before I got it all in a box.

Good luck dude!

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